U.S. Pat. No. 4,959,765 in the name of the Applicant describes a layout corresponding to a resonant converter which switches at zero voltage and at zero current.
In particular, that patent describes a push-pull type device in which the resonant circuit is constituted by a current feed capacitor and by the leakage inductance of the transformer.
That converter is not protected against an accidental short circuit of one of the switches in the push-pull version, or of the sole switch in the other version described.
To obtain a zero voltage switch, the switch of one push-pull branch should be closed only after the switch in the other branch has been opened and the voltage across the terminals of said switch has reached its high value due to the magnetization energy of the transformer, and has reached its low value at the switch which is to be closed under the action of the transformer.
U.S. Pat. 5,140,512 (O'Sullivan) enables such switching at zero voltage to be obtained automatically by detecting the voltage excursion of the transformer. A detection logic circuit is associated with each push-pull switch and alternating push-pull operation is authorized only when the transformer has switched within a time interval during which both push-pull switches are open.
Finally, U.S. Pat. No. 5,249,113 in the name of the Applicant describes a converter having a transformer which enables a rapid transition to be obtained and provides good operation for the layout when the switches are insulated grid field effect transistors (MOSFET) which present high drain-source capacitance tending significantly to slow down the automatic switching of the transformer, or which are responsible for an unfavorable form factor of the current and for poor efficiency. Adjusting the magnetization current of the transformer which does not change the resonance characteristic of the transformer serves to increase the switching speed of the transformer, thereby improving the form factor and the efficiency.
By implementing the three above-cited patents simultaneously, it is possible to make converters having efficiency of 97% or more on a bus regulated at 50 V. Such converters nevertheless require input protection in the event of a short circuit fault, which gives rise to greater complexity.